This proposal addresses two issues important to the study of mental health: the effects of the maternal/fetal interaction on the development of behavior and the expression of a biologically, socially and psychologically consequential behavior, maternal responsiveness. Exposure to gonadal steroid hormones during prenatal life has dramatic consequences on the development and sexual differentiation of the brain and behavior. However, fetuses are also exposed to maternal circulating hormones which undergo rapid and extreme changes prior to parturition. Yet, the developmental behavioral effects of fetal exposure to maternal hormones remains completely unstudied. While the development of sexual behavior and the effects of prenatal androgens is well understood, the development of another critical hormonally-mediated reproductive behavior, maternal behavior, has not been well examined. The main objective of this proposal is to determine whether the unique pattern of maternal steroid hormones plays a significant role in the development of maternal behavior. The excellent model of maternal behavior, the parturient rat, will be used in these studies. Specific Aim I will examine when during prenatal life the fetal brain becomes sensitive to gonadal steroid hormones. Using immunocytochemistry and in situ hybridization, it will be determined when neurons in the fetal brain first begin to express steroid hormone receptors. Although it is known that steroid hormones given to pregnant females can alter the development of the fetus, it has yet to be demonstrated empirically that hormones in the maternal circulation enter the fetal brain. Specific Aim II will use steroid hormone autoradiography to determine if radiolabelled steroid hormones in maternal circulation can cross the placenta and the fetal blood/brain barrier to bind to nuclear steroid receptors within the fetal brain. To address directly, the effects of the maternal pregnancy hormones, estrogen and progesterone on the development of maternal behavior, Specific Aim III will use a method in which pregnancy is maintained in an ovariectomized female by hormone administration. In this way, the changes in estrogen and progesterone that normally occur during pregnancy can be mimicked and manipulated. The effects of these manipulations on the expression of maternal behavior in the adult, female offspring will be examined. The results from these experiments will provide insight as to the importance of the maternal environment on the development of a behavior that is key to mental health, maternal care.